1. Field of the Invention
The present invention relates to alumina compositions for coating catalyst supports, to process for the preparation thereof and to the catalyst supports prepared therefrom. The invention more especially relates to alumina compositions for coating metallic or ceramic substrates to fabricate catalyst supports having a surface layer possessing both microporosity and macroporosity.
2. Description of the Prior Art
It is known to this art that catalysts comprised of a substrate coated with a film or layer of refractory oxides upon which a catalytically active phase is deposited, are useful in a wide variety of gaseous phase reactions. Thus, the catalytic purification or conversion of the exhaust gases from gasoline or diesel fueled internal combustion engines is at the present time effected by means of catalysts comprising a ceramic or metallic substrate in the form of an inert and rigid (monolithic) structure in a honeycomb configuration, coated with a film or layer of alumina, and onto which the active catalyst phase is deposited/coated.
The coating of ceramic monoliths with a film of refractory oxides, and in particular of alumina, is relatively easy, but the coating of metallic monoliths is much more difficult and requires, for example, the use of surface-oxidized steels containing aluminum, such that the refractory oxide or alumina layer may subsequently be more effectively attached.
Heretofore, there existed no composition that could be indiscriminantly coated onto both ceramic and metallic substrates without the necessity for a preliminary treatment of such substrates and which would provide an excellent bond between the ceramic or metallic substrate and the film or layer of the refractory oxide coating.
Alumina compositions for the coating of catalyst supports too are known to this art; compare U.S. Pat. No. 3,554,929. The alumina compositions described in this patent contain 5 to 10% colloidal boehmite and 90 to 95% of activated alumina particles having a specific surface area of 0 to 600 m.sup.2 /g and a particle size between 8 and 60 microns. According to Example 1 of the patent, it is necessary, in order to coat an aluminum shaped body (Alloy 5052 containing 2.5% magnesium) with the alumina composition described, to initially coat the shaped body with a layer containing principally mullite, a high proportion of crystalline alumina and a small amount of amorphous material.
Also according to the aforesaid '929 patent, the alumina compositions with large specific surface areas claimed therein cannot be used to directly coat metallic surfaces without a preliminary treatment. Furthermore, the alumina coatings with large specific surface areas obtained according to this patent do not have satisfactory porosity properties; in fact, the coatings obtained are microporous and do not have the macroporosity required for good catalytic effectiveness.